1. Field of the Invention
This invention relates to an identification token for use with an electronic identification/actuation system and more particularly, this invention relates to a positive identification system wherein a coded transponder is energized by radio frequency interrogation to produce a coded identification signal which identifies the person or object bearing the transponder to selectively control access to a restricted space.
2. Description of the Prior Art
It is known in the applicable art to employ identification systems that utilize a variety of identification means. Radio frequency means allow identification without the necessity of physical contact between the interrogator and the transponder, but such systems typically require the use of battery powered transponders or large antenna geometries to attain reasonable distances between the interrogator and transponder. Many of these systems employ complex interrogation schemes in order to attain the radio frequency power levels necessary to properly excite a passive, i.e. non battery powered transponder.
Electronic locking means are known in the art in which there is an interaction between the key part and the lock part. Such systems are already known and can be used, for example, in case certain locations in a building are accessible only to a predetermined class of persons. The persons carry an identification token comprising an electrical circuit which generates a signal in response to an electromagnetic field generated by an oscillator housed in or near the lock. Alternatively, the token may modify the strength of the oscillator's electromagnetic field which can be detected by suitable means.
In prior art recognition and identifier systems there are two directions or communications between the reader and the identifier devices. The reader (recognition station) which is typically stationary, sends an interrogation signal and/or power to the identifier device (e.g. a card, a tag, or a key) which is typically portable. In response the identifier device sends a coded signal bearing intelligence about its identification to the reader. Means must be provided so that the two directions of the communication in power do not interfere to the extent of inhibiting the desired transfers. One characteristic of these systems is that inductive coupling is often used between the reader and the identifier; another characteristic is that often both power and data are transmitted over the same inductively coupled coils and great care must be taken to prevent interference between the token and reader.
Walton, in U.S. Pat. No. 4,388,524, describes an electronic identification and recognition system in which the recognition portion includes a voltage controlled oscillator which sweeps over a range of frequencies. The token, or identification portion, resonates at certain frequencies determined by a variable reactance in the resonant circuit in the token which is responsive to bits from a sequential data register. The bits result in the on/off switching of the variable reactance which modifies the resonant frequency in the token or modifies the frequency of an oscillator in the token. The changes and sequence of changes in the resonant frequency are detected and recorded by the recognition portion such that the internal code of the token or identifier is determined.
Kreft, in U.S. Pat. No. 4,602,253, describes an apparatus for mutual information transmission between an electronic lock and key using paired antenna coils at both the key and lock. A periodic signal is emitted from the lock and is received by the key. Upon activation of the key electronics and emission of a coded information signal. This return signal received from the lock coil is briefly short-circuited or damped at the key side in order to produce a synchronization switch synchronizing the points in time of the signal appearance. Electronic converters are provided in the reader which, given the precondition that the short-circuited signal extends over a plurality of signal pulses, generate a digital signal corresponding to the short circuit times.
Kip et al, in U.S. Pat. No. 4,196,418, describes a detection plate or identification key for an identification system comprising a resonant circuit, a switching device, a coding means and a separate pulse generator fed by the resonant circuit, the output of the pulse generator being connected to the coding means. Kip et al's invention requires the presence of a separate pulse generator in the token circuit. It would be desirable to provide an identification token that did not require the presence of a separate pulse generator inside the token for regulating coding means. The Kip et al detection plate (or token) circuit includes a resonant circuit comprising a coil and capacitor in parallel; rectifier means and capacitors which form a supply circuit to provide a supply voltage when the resonant circuit is in oscillation; a digital counter which receives supply voltage from the supply circuit associated with the resonant circuit; two digital multiplex circuits having control encoding inputs connected to outputs of the counter and a switching device such as a transistor connected to the outputs of the multiplex circuits which is brought into conductive and nonconductive states by means of the code signal from the multiplex circuits. The switching device short-circuits the resonant circuit to form a coded signal. However, since the resonant circuit also supplies supply voltage for the circuitry of the detection plate, it cannot remain short-circuited for too long a time. A signal from the counter supplied to the coding inputs of the multiplex circuits insures that the resonant circuit is not short-circuited by the transistor for at least 50% of the time so that the energy transfer blocks are formed in resonant circuit signal.
In U.S. Pat. No. 4,510,495, issued to Sigrimis et al, an interrogator's power transmitter, using a loop antenna, transmits a plurality of radio frequency burst signals to power the transponder; the between-burst time periods are used so that the interrogator receiver section can "listen" to the transponder without being overloaded by the transmitter. The transponder, although powered remotely by the power transmitter, transmits a pulse-modulated carrier signal back to the receiver, which includes a preamplifier switching system to avoid receiver damage.
U.S. Pat. No. 4,333,072 to Beigel discloses a close-coupled identification system wherein a tag (passive transponder) is brought in close proximity to a probe circuit to verify identification. Beigel discloses a probe circuit which is loaded and unloaded by an identification device in a predetermined time sequence. This probe circuit uses a push-pull amplifier arrangement, along with an autotransformer coil, to generate an alternating magnetic field.
The foregoing systems require the use of fairly high energy to be transmitted to properly power the tag circuit or transponder. The elimination of the use of circuitry such as push-pull amplifiers and receiver input preamplifier switching is desirable from both cost and physical size standpoints. In addition, the simplification of the interrogation circuitry and improvement in the receiving circuitry sensitivity would result in advantages over the existing art.
Other miscellaneous identification tokens having integral power supplies such as a battery are well known in the literature. For example, Battry in U.S. Pat. No. 4,353,064, describes a battery-operated access control card. The card receives a code from the reader. It checks the code to see if it matches its memory. If so, it transmits a second code. The card has no LC circuit.